1. Field of The Invention
The present invention generally relates to a pallet stacking type storage system, and more specifically to such a system which is particularly suitable for storing cassettes or boxes in which semiconductor wafers (for example) are disposed.
2. Description of the Prior Art
In order to store semiconductor wafers, it is a common practice to place them in cassettes or containers which are then deposited in a clean environment.
Where the wafers to be stored are small in number, the cassettes can be simply placed on fixed shelves. However, where the number of wafers to be stored is very large, two major drawbacks are encountered, i.e., considerable floor space is required and the cassettes located in high storage positions are difficult to reach, inducing handling difficulties.
One proposal for overcomming the above difficulties has taken the form of storing the wafers on a plurality of movable shelves which are permanently coupled to endless chains carried on vertically arranged sprocket wheels.
This prior art however, has a drawback in that as the chain must be sufficiently slack to ensure smooth circulation thereof. Hence precise positioning of each shelf becomes difficult, rendering this system unsuitable for use in an automatic working system. Furthermore, the chain and sprockets of this system requires lubricant which tends to pollute the clean environment in which the system is located.